This section provides background information to facilitate a better understanding of the various aspects of the invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
The invention relates generally to devices for use in detonating an explosive device and more specifically to a ballistic transfer delay device to introduce a time delay in a ballistic train.
Explosive charges are utilized in wellbores to perform various functions, for example, for perforating wells, for formation testing, to cut equipment, and to actuate devices such as bridge plugs, anchors, valves and packers. Heretofore, various time delay devices have been utilized to offset the time at which two or more explosive devices disposed in the well are detonated. For example, it is often desired to detonate multiple perforating gun sections to perforate the well casing and/or formation surrounding the wellbore. In particular, it is often desired to perforate different zones (i.e., sections) in a wellbore that are spaced a distance apart from one another. In these operations, a perforating gun may be run into the wellbore and the first zone perforated, the operator then pulls out of the wellbore and runs into the well with a second perforating gun to perforate the second wellbore zone. To eliminate multiple trips into the wellbore it is known to create extended length bottomhole assemblies (“BHA”) that utilized blank spacer sections to separate the perforating gun sections. Drawbacks of this type of operation include increased time and expense to make-up the extended BHA length; the increased size of the BHA may require the use of larger surface drilling units (i.e., rigs, workover equipment) than desired or than is available; and the needed BHA length may exceed the length limitation for entering the wellbore through a lubricator.
To alleviate the use of extended length perforating gun assemblies, the spacer gun sections have been replaced with firing heads for each zone to be perforated, thus reducing the overall length and weight of the BHA. Each of the firing heads, for example hydraulic delay firing heads (“HDF”), can be initiated at the same time and the hydraulic time delay for the various HDFs is staggered using different coefficient orifices. When the delay of the first firing head expires a mechanical actuator (e.g., hydraulic piston, firing pin) impacts a detonator which fires for example into the input booster of the associated gun section. Upon firing of the first gun section, the BHA must be moved to the second perforation zone during the delay of the second firing head. This process repeats for the number of zones to be perforated. These operation require that the BHA be moved from zone to zone during the predetermined time delay of the respective firing heads, thus requiring that the delay time for each firing head and the cumulative delay times must be accurately calculated to achieve a successful operation. Pressure drops can occur from the time one gun section perforates and the next delay firing head operates. These pressure drops affect the delay time of all the unfired heads and must be compensated for when determining the detonation of the subsequent gun sections.
Another technique is the use of multiple hydraulic delay firing heads with ballistic delay charges. In these operations, the detonation of a first perforating gun (i.e., gun section) initiates the adjacent firing head causing a mechanical device (e.g., hydraulic piston, firing pin) to impact a detonator that ignites a delay fuse which burns and then transfers the burn to detonate the associated perforating gun section(s). The next delay firing head is initiated by the pressure of the detonated adjacent perforating gun, causing its firing pin to impact the detonator and ignite the delay fuse. The BHA is moved to the next wellbore zone during the time delay. These systems are pressure activated and require that the firing pins seal after activation of the firing pin to prevent pressure from above the particular firing head from communicating with the delay charge and gun (i.e., detonation cord and explosive charges) below the firing head. Communication of a pressure leak from above the firing head and delay charge can result in detonation of the gun off of the desired zone.
It is therefore a desire to provide a ballistic transfer delay device that can provide time delay in the ballistic transfer train. It is a further desire to provide a ballistic transfer delay device that does not have a firing pin. It is a still further desire to provide a ballistic transfer delay device that does not have a hydraulic firing mechanism. It is still a further desire to provide a ballistic transfer delay device that does not have a pressure requirement to be initiated.